"Right up to these enormous catchments that are so important for irrigation — for our food bowls in Australia and the rest of the world."

Going national

At a backyard level it is easy to figure out, but across the country it gets a bit trickier.

According to Dr Henley it takes a process called topographic analysis — the study of the shape of the surface of the earth (so, where the hills are).

"Geoscience Australia and the Bureau of Meteorology and various other agencies keep records and update these records of the topography of the country, and these topographic maps can be used to define [catchment] boundaries," he said.

Because they know where the hills are, the bureau, along with other science agencies, have been able to work out where the water will tend to go wherever it falls in Australia.

Andrew Preece, senior hydrologist with the Bureau of Meteorology's flood forecasting team, said widespread heavy rainfall in early March brought several hundred millimetres of rain in the Georgina, Diamantina and Coopers Creek systems, which are part of the Lake Eyre Basin.

So why is the beginning of March still significant a month later in April?

"The river systems out there are fairly flat, fairly slow moving," Mr Preece said.

"We had the rainfall at the beginning of March and we're only just starting to see the flood peak approaching the Queensland border."